Conversion of low molecular weight alkanes, such as methane, to synthetic fuels or chemicals, has received increasing attention as low molecular weight alkanes are generally available from secure and reliable sources. For example, natural gas wells and oil wells currently produce vast quantities of methane. In addition, low molecular weight alkanes are generally present in coal deposits and may be formed during mining operations, in petroleum processes, and in the gasification or liquefaction of coal, tar sands, oil shale, and biomass.
Many of these alkane sources are located in relatively remote areas, far from potential users. Accessibility is a major obstacle to effective and extensive use of remotely situated methane, ethane and natural gas. Costs associated with liquefying natural gas by compression or, alternatively, constructing and maintaining pipelines to transport natural gas to users are often prohibitive. Consequently, methods for converting low molecular weight alkanes to more easily transportable liquid fuels and chemical feedstocks are desired and a number of such methods have been reported.
Reported methods can be conveniently categorized as direct oxidation routes and/or as indirect syngas routes. Direct oxidative routes convert lower alkanes to products such as methanol, gasoline, and relatively higher molecular weight alkanes. In contrast, indirect syngas routes involve, typically, production of synthesis gas as an intermediate
As is well known in the art, synthesis gas ("syngas") is a mixture of carbon monoxide and molecular hydrogen, generally having a dihydrogen to carbon monoxide molar ratio in the range of 1:5 to 5:1, and which may contain other gases such as carbon dioxide. Synthesis gas has utility as a feedstock for conversion to alcohols, olefins, or saturated hydrocarbons (paraffins) according to the well known Fischer-Tropsch process, and by other means. Synthesis gas is not a commodity; rather, it is typically generated on-site for further processing. One potential use for synthesis gas is as a feedstock for conversion to high molecular weight (e.g. C.sub.50+) paraffins which provide an ideal feedstock for hydrocracking for conversion to high quality jet fuel and superior high cetane value diesel fuel blending components. Another potential application of synthesis gas is for large scale conversion to methanol.
In order to produce high molecular weight paraffins in preference to lower molecular weight (e.g. C.sub.8 to C.sub.12) linear paraffins, or to synthesize methanol it is desirable to utilize a synthesis gas feedstock having an H.sub.2 :CO molar ratio of about 2:1 or less. As is well known in the art, Fischer-Tropsch syngas conversion reactions using syngas having relatively high H.sub.2 :CO ratios produce hydrocarbon products with relatively large amounts of methane and relatively low carbon numbers. For example, with an H.sub.2 :CO ratio of about 3, relatively large amounts of C1-C8 linear paraffins are typically produced. These materials arc characterized by very low octane value and high Reid vapor pressure, and are highly undesirable for use as gasoline.
Lowering the H.sub.2 :CO molar ratio alters product selectivity by increasing the average number of carbon atoms per molecule of product, and decreases the amount of methane and light paraffins produced. Thus, it is desirable for a number of reasons to generate syngas feedstocks having molar ratios of hydrogen to carbon monoxide of about 2:1 or less.
Prior methods for producing synthesis gas from natural gas (typically referred to as "natural gas reforming") can be categorized as (a) those relying on steam reforming where natural gas is reacted at high temperature with steam, (b) those relying on partial oxidation in which methane is partially oxidized with pure oxygen by catalytic or non-catalytic means, and (c) combined cycle reforming consisting of both steam reforming and partial oxidation steps.
Steam reforming involves the high temperature reaction of methane and steam over a catalyst to produce carbon monoxide and hydrogen. This process, however, results in production of syngas having a high ratio of hydrogen to carbon monoxide, usually in excess of 3:1.
Partial oxidation of methane with pure oxygen provides a product which has an H.sub.2 :CO ratio close to 2:1, but large amounts of carbon dioxide and carbon are co-produced, and pure oxygen is an expensive oxidant.
An expensive air separation step is required in combined cycle reforming systems, although such processes do result in some capital savings since the size of the steam reforming reactor is reduced in comparison to a straightforward steam reforming process.
Thus, it is desirable to lower the cost of syngas production as by, for example, reducing the cost of the oxygen plant, including eliminating the cryogenic air separation plant, while improving the field as by minimizing the co-production of carbon, carbon dioxide and water, in order to best utilize the product for a variety of downstream applications.
In view of the great commercial interest in preparing synthesis gas by reforming readily available hydrocarbon feedstocks such as natural gas, and because of the benefits of conducting these reforming reactions in the presence of a catalyst that remains active for an extended period of use, there is a continuing need for new, less expensive, durable, coke resistant, more active and selective catalysts for the production of synthesis gas. The present invention provides such catalysts as well as a method for preparing synthesis gas using such catalysts.
European Patent Application 90305684.4, published on Nov. 28, 1990, under Publication No. EP 0 399 833 Al in the name of Cable et al., describes an electrochemical reactor using solid membranes comprising; (1) a multi-phase mixture of an electronically-conductive material, (2) an oxygen ion-conductive material, and/or (3) a mixed metal oxide of a perovskite structure. Reactors are described in which oxygen from oxygen-containing gas is transported through a membrane disk to any gas that consumes oxygen. Flow of gases on each side of the membrane disk in the reactor shell shown are symmetrical flows across the disk, substantially radial outward from the center of the disk toward the wall of a cylindrical reactor shell. The gases on each side of the disk flow parallel to, and co-current with, each other.
Materials known as "perovskites" are a class of materials which have an X-ray identifiable crystalline structure based upon the structure of the mineral perovskite, CaTiO.sub.3. In its idealized form, the perovskite structure has a cubic lattice in which a unit cell contains metal ions at the corners of the cell, another metal ion in its center and oxygen ions at the midpoints of each cube edge. This cubic lattice is identified as an ABO.sub.3 -type structure, where A and B represent metal ions. In the idealized form of perovskite structures, generally, it is required that the sum of the valences of A ions and B ions equal 6, as in the model perovskite mineral, CaTiO.sub.3.
Many materials having the perovskite-type structure (ABO.sub.3 -type) have been described in recent publications including a wide variety of multiple cation substitutions on both the A and B sites said to be stable in the perovskite structure. Likewise, a variety of more complex perovskite compounds containing a mixture of A metal ions and B metal ions (in addition to oxygen) are reported. Publications relating to perovskites include: P. D. Battle et al., J. Solid State Chem., 76,334 (1988); Y. Takeda et al., Z Anorg. Allg. Chem., 550/541,259 (1986); Y. Teraoka et al., Chem. Lett., 19, 1743 (1985); M. Harder and H. H. Muller-Buschbaum, Z. Anorg. Alig. Chem., 464, 169 (1980); C. Greaves et al., Acta Cryst., B31,641 (1975).
U.S. Pat. No. 5,126,499 in the names of Takashi Hayakawa, Katsuomi Takehira, Hideo Orita, Masao Shimizu and Yoshihito Watanabe (Hayakawa et al.) and assigned to Director-General of Agency of Industrial Science and Technology, Japan, describes a process for the production of hydrocarbons by oxidative coupling of methane with an oxide of metals having the following composition: EQU M.sub.1 (Co.sub.1-x Fe.sub.x).sub.1 O.sub.y
i.e. a perovskite-type, because the ratio of B positioned metal ions and A positioned metal ions equal 1, and is described as such (Hayakawa et al., column 2, lines 25 to 39).
United Kingdom Patent Application GB 2213496 A listing Lywood as inventor describes the production of hydrogen-containing gas streams by an endothermic catalyzed reforming between methane and steam. The '496 Application proposes the following equations for the steam reforming of methane: EQU CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2 1. EQU CH.sub.4 +2H.sub.2 O.fwdarw.CO.sub.2 +4H.sub.2 2. EQU CH.sub.4 +CO.sub.2 .fwdarw.2CO+2H.sub.2 3.
U.S. Pat. No.4,592,903 issued to Osman et al., states that carbon monoxide can be exothermically converted to carbon dioxide and hydrogen through a reaction termed a water-gas shift, represented by the equation: EQU CO+H.sub.2 .fwdarw.CO.sub.2 +H.sub.2 4.
Reportedly, the "shift" reaction, can be accomplished in two shift conversion vessels operating at different temperatures to maximize yield. The '903 patent states that a temperature of from about 600.degree. to 900.degree. F. and a pressure of about 300 psig to 1,000 psig is effective in a high-temperature shift converter containing a supported, chromium-promoted iron catalyst. The '903 Patent further states that a low-temperature shift conversion takes place over a catalyst comprising a mixture of zinc and copper oxides at a temperature of from about 400.degree. to 500.degree. F. and a pressure of from about 300 psig to about 1,000 psig.
It is important to distinguish between the steam reforming of hydrocarbons, as described above, and the partial oxidation of hydrocarbons. Partial oxidation of methane produces two moles of dihydrogen (diatomic hydrogen) for each mole of methane reacted. In contrast, steam reforming of methane produces three moles of dihydrogen per mole of reacted methane.
Partial oxidation of methane is described, for example, in U.S. Pat. No. 4,618,451 issued to Gent. The '451 Patent states that methane is reacted with oxygen from an air separation plant, the proportion of oxygen being less than sufficient for complete combustion. A hot gas containing hydrogen and carbon monoxide is said to be produced. The '451 patent also states that steam or nitrogen can be present during the combustion to act as a temperature modifier and to avoid soot formation. Additional hydrocarbon is, reportedly, injected into the hot gas, and the resulting gas mixture is reacted over a steam reforming catalyst.
A particular class of partial oxidation processes for converting methane or natural gas to synthesis gas are known as autothermic processes. By convention, the autothermic process includes an exothermic oxidation step and an endothermic steam reforming step which are in approximate heat balance. For example, U.S. Pat. No. 5,112,257 issued to Kobylinski and assigned to the assignee of the present invention, describes an autothermal process for converting natural gas to synthesis gas which includes the steps of mixing natural gas with air, subjecting a resulting mixture to simultaneous partial oxidation and steam reforming reactions, and subsequently reacting unconverted alkanes with water in the presence of a catalyst having steam reforming activity.
Processes which produce hydrogen or hydrogen-containing mixtures by reacting a single-carbon saturated alcohol, methanol, with steam are collectively termed methanol steam reforming processes. U.S. Pat. No. 4,091,086, issued to Hindin et al., describes a process for producing hydrogen by reacting steam with methanol in the presence of a catalytic composition at elevated temperatures. The '086 Patent states that methanol can be converted to hydrogen in a single-stage reaction over a catalytic composition comprising zinc oxide, copper oxide, thorium oxide, and aluminum oxide. Moreover, the '086 Patent states, without citing authority or presenting evidence in support, that the composition catalyzes a purported methanol decomposition. The purported decomposition is described as producing significant amounts of carbon monoxide which are immediately consumed in a water gas shift reaction.
U.S. Pat. 3,791,933, issued to Rostrup-Nielsen, discloses the preparation of catalysts for reforming gaseous or vaporizable liquid hydrocarbons using steam, carbon oxide, oxygen and/or air. Examples in U.S. Pat. No. 3,791,993 show that compositions having nickel, magnesium and aluminum are suitable for converting naphtha to hydrogen-rich gaseous products using steam reforming.
U.S. Pat. No. 4,743,576, issued to Broecker et al., describes a catalyst for the production of synthesis gas or hydrogen from aqueous methanol by dissociation or steam reforming. The catalyst reportedly contains a noble metal component on an oxide carrier which comprises an oxide of cerium or titanium and, also, an oxide of zirconium or lanthanum.
Hydrotalcite-like compounds have been used as catalysts in a variety of applications, such as catalysts for aldol condensation, polymerization of alkene oxides, hydrogenation catalysts, dehydrogenation catalysts, etc., as described in F. Cavani et al., Catalysis Today, Volume 11, pages 173-301, 1991. Cavani et al. discloses that coprecipitated Ni, Al-based catalysts have been recognized as satisfying all the requirements for operation in steam reforming for methane production, and that coprecipitated catalysts calcined at 723.degree. K. (450.degree. C.) and reduced at 723.degree. K. were active in the 673.degree. K. to 923.degree. K. (450.degree. C. to 650.degree. C.) range for steam cracking of naphtha to produce methane. U.S. Pat. No. 3,865,753 to Broecker et al. discloses the use of a catalyst prepared by calcining [Ni.sub.5 MgAl.sub.2 (OH).sub.16 ]CO.sub.3 .multidot.4H.sub.2 O at a temperature in the range of 350.degree. C. to 550.degree. C., and which is subsequently reduced with hydrogen. Such a catalyst was used for the steam cracking of hydrocarbons having 2 to 30 carbon atoms at a temperature in the range of 300.degree. C. to 450.degree. C. to form methane.
Ross et al., J. of Catalysis, Volume 52, pages 280-290, 1978, have examined the reaction of methane with water over a catalyst derived from Ni.sub.6 Al.sub.2 (OH).sub.16 CO.sub.3 .multidot.4H.sub.2 O at temperatures of 873.degree. K. to 973.degree. K. (600.degree. C. to 700.degree. C.). Kruissink et al., J. Chemical Society, Faraday Trans. I, Volume 77, 649-663, 1981, discusses the thermal treatment of nickel-containing compositions having x-ray patterns characteristic of the hydrotalcite-like minerals; and Hernandez et al., Thermochemica Acta, Volume 81, 311-318, 1984, investigated the thermal decomposition of hydrotalcite-like compounds of formula [Ni.sub.(1-s) Al.sub.x (OH).sub.2 ].sup.x+An.sub.x/n .multidot.mH.sub.2 O where A is carbonate and sulfate. Using x-ray diffraction studies, these researchers identified nickel oxide as the decomposition product at temperatures above 600.degree. C., whereas the corresponding spinel, NiAl.sub.2 O.sub.4, was formed at temperatures higher than 1000.degree. C.
British Patent 1,342,020, discloses catalysts having chemical composition Ni.sub.6 Al.sub.2 CO.sub.3 (OH).sub.16 .multidot.4H.sub.2 O and Ni.sub.3 Mg.sub.3 Al.sub.2 CO.sub.3 (OH).sub.16 .multidot.4H.sub.2 O and discloses that they have an application as hydrogenation, dealkylation and cracking catalysts. Clause et al, J. of Catalysis, Volume 133, 231-246 (1992) discloses the preparation and analysis of nickel-aluminum mixed oxides obtained by thermal decomposition of hydrotalcite-type precipitates. This paper also discloses that nickel-aluminum mixed oxides resulting from the thermal decomposition of hydrotalcite-like coprecipitates have been studied for steam reforming and methanation reactions.
Commonly assigned U.S. Pat. No. 5,399,537, issued to Bhattacharyya, Chang, Kleefisch and Udovich, discloses a nickel-containing catalyst precursor composition comprising at least one hydrotalcite-like compound having a pre-selected formula. Catalyst compositions formed therefrom by heat treatment to elevated temperatures under reforming conditions are, particularly, useful for the production of synthesis gas and which catalysts are resistant to coke formation when used to catalyze the reaction of a hydrocarbyl compound with an oxygen-containing gas at elevated temperatures to form synthesis gas. Also see, for example, commonly assigned U.S. Pat. No. 5,921,238. Patents '537 and '238 are specifically incorporated herein in their entirety by reference.
It is an object of the invention to overcome one or more of the problems described above.
Other objects and advantages of the invention will be apparent to those skilled in the art from a review of the following detailed description, taken in conjunction with the drawing and the appended claims.